1. Field of the Invention
The invention relates to an exhaust gas recirculation system and exhaust gas recirculation method for an internal combustion engine.
2. Description of the Related Art
Japanese Patent Application Publication No. 2005-146960 describes the following technology. In a predetermined period that is close to the time at which the combustion state in an internal combustion engine is changed, an amount of external EGR gas is adjusted by an external EGR unit so that the amount of external EGR gas that is supplied into a cylinder via the external EGR unit matches a predetermined EGR gas amount that corresponds to the combustion state after the combustion state is changed. In addition, when the amount of external EGR gas is smaller than the amount of EGR gas that should be supplied into the cylinder in the predetermined period, a portion of combustion gas is supplied as internal EGR gas into the cylinder by adjusting the opening/closing timing of the intake valve and the exhaust valve in order to cover the shortage of the EGR gas.
When the opening/closing timing of the intake valve and the exhaust valve is changed by a large amount to adjust the amount of internal EGR gas by a large amount, the temperature in the cylinder fluctuates widely. As a result, unburned HC that is discharged from the internal combustion engine excessively increases or smoke is generated, resulting in deterioration of exhaust emission.